The present invention relates to a method and to an apparatus for producing spread-coded signals, in particular spread-coded mobile radio signals.
Telecommunication is one of the fastest developing technologies. In the mobile radio field, work is currently being carried out on the standardization and development of what is referred to as third-generation mobile radio, which is referred by the term UMTS (Universal Mobile Telecommunications System) and envisages a world-wide uniform mobile radio standard.
According to the UMTS mobile radio standard, what is referred to as CDMA (Code Division Multiple Access) is used as the multiple access method. This is a multiple access method in which all the subscribers to the corresponding mobile radio system are allowed to use the entire available system bandwidth at the same time. In order to make it possible, despite this, to avoid collisions between the individual subscribers, the individual subscriber signals are provided with different code sequences which thus allow unique association of the received signals. During this process, the individual signals are spread, as a result of which the bandwidth is multiplied, so that this technique is also referred to as the spread-spectrum technique. In particular, the subscriber data that will be transmitted is multiplied or spread using a code sequence which is independent of the data that will be transmitted. The band-spread signal obtained in this way is then modulated onto a radio-frequency carrier, and is transmitted to a corresponding receiver. The receiver demodulates this band-spread signal and carries out despreading, using a code sequence which is synchronized to the transmitter. The receiver admittedly receives not only the desired signal from the transmitter, but also additional signals from other transmitters, transmitting in the same frequency band. However, the despreading process ensures that the only signal which is despread and whose bandwidth is reduced is that which uses the same and synchronous spread code as the receiver, so that, after the despreading process, the desired signal can easily be filtered out.
While transmission apparatus for digitally producing mobile radio transmission signals for different mobile radio standards, in particular, for example, for the currently normally used GSM (Global System for Mobile Communication) mobile radio standard, are already known, the problem of producing spread-modulated signals in mobile terminals for third-generation mobile radio has not yet been solved.
The UMTS standard xe2x80x9cUltra physical layer description FDD partsxe2x80x9d, Ovesjxc3x6 Fredrik, European Telecommunication Standard, XX, XX, Jun. 25, 1998 (1998-06-25), pages 1 to 41, describes a method for producing spread-coded signals, in which a signal is subjected to spread coding and to subsequent scrambling. The band-spread and scrambled signal is then filtered in an RRC filter.
The technical article xe2x80x9cTransmit pulse shaping filters and CORDIC algorithm based precompensation for digital satellite communicationsxe2x80x9d, M. Vandermaar et al., IEEE Proceedings of the 39th Midwest Symposium on Circuits and Systems (Cat. No. 96CH35995), AMES, IA, USA, Aug. 18-21, 1996, pages 1219 to 1222, Vol. 3, 1996, New York, N.Y., USA, IEEE; USA ISBN: 0-7803-3636-4) describes an algorithm for precompensation for non-linearities that occur in a signal amplifier. The compensator has two CORDIC (Coordinate Rotation Digital Computer) stages, which carry out a coordinate transformation on the signal from cartesian coordinates to polar coordinates, and vice versa. The computation steps for the actual signal compensation are carried out between the two CORDIC stages.
U.S. Pat. No. 5,838,733 describes a transmitting device, in which a digital signal that will be transmitted is subjected to programmable amplification and to amplitude limiting.
It is accordingly an object of the invention to provide an apparatus and a method for producing spread-coded signals which overcome the above-mentioned disadvantages of the prior art apparatus and methods of this general type.
In particular, it is an object of the invention to provide an apparatus and a method for producing spread-coded signals that can be used, in particular, in third generation mobile radio transmitters.
Furthermore, it is an object of the invention to enable spread-coded signals to be produced in accordance with different mobile radio standards, for example, in accordance with the UMTS or IS-95 standard.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing spread-coded signals that includes steps of: coding and band-spreading transmission data using a spread code to obtain band-spread transmission data; coding the band-spread transmission data using a scrambling code to obtain band-spread and scrambled transmission data and then pulse shaping the band-spread and scrambled transmission data; subjecting the band-spread and scrambled transmission data to frequency correction; and in a signal path downstream from the frequency correction, subjecting the band-spread and scrambled transmission data to an operation selected from the group consisting of a DC offset operation and an amplitude compensation operation.
In accordance with an added mode of the invention, the pulse shaping is performed by Root Raised Cosine filtering using a roll-off factor of approximately 0.22.
In accordance with an additional mode of the invention, during the pulse shaping, the band-spread and scrambled transmission data is oversampled using a specific oversampling factor.
In accordance with another mode of the invention, the oversampling factor is four.
In accordance with a further mode of the invention, digital frequency offset correction is used for the frequency correction; a rotation angle is used for the frequency correction that is represented as a linear combination of microrotations; and the microrotations are performed as a function of a frequency offset by selectively using a CORDIC algorithm.
In accordance with a further added mode of the invention, the method includes: performing both the DC offset operation and the amplitude compensation operation; performing the DC offset compensation by adding a specific DC offset compensation value to the band-spread and scrambled transmission data; and performing the amplitude compensation by multiplying the band-spread and scrambled transmission data by a specific amplitude compensation value.
In accordance with a further additional mode of the invention, before the transmission data is coded using the spread code, the transmission data is weighted by multiplying the transmission data by a weighting factor. Subsequently, the transmission data is oversampled.
In accordance with yet an added mode of the invention, the method includes using an over-sampling factor to perform the step of oversampling the transmission data such that when the subsequent step of coding the transmission data using the spread code is performed, a maximum chip rate is obtained.
In accordance with yet an additional mode of the invention, the method includes performing the step of coding and band-spreading the transmission data, the step of coding the band-spread transmission data using the scrambling code, the step of pulse shaping the band-spread transmission data, the step of frequency correction, and the step of oversampling the transmission data by:
obtaining the transmission data from a plurality of physical channels, each providing first transmission data;
for each one of the plurality of the physical channels and separately from others of the plurality of the physical channels, weighting, oversampling, and coding the first transmission data using a channel-specific spread code;
after being coded using the spread code, combining the first transmission data of the plurality of the physical channels to form an I branch carrying second transmission data and a Q branch carrying third transmission data;
scrambling the second transmission data, and then pulse shaping the second transmission data, frequency correcting the
second transmission data, and subjecting the second transmission data to an operation selected from the group consisting of a DC offset compensation operation, and an amplitude compensation operation; and
separately from the second transmission data, scrambling the third transmission data, and then pulse shaping the third transmission data, frequency correcting the third transmission data, and subjecting the third transmission data to an operation selected from the group consisting of a DC offset compensation operation, and an amplitude compensation operation.
In accordance with yet an additional mode of the invention, the method includes using a modulator to perform the step of coding and band-spreading the transmission data, the step of coding the band-spread transmission data using the scrambling code, the step of pulse shaping the band-spread transmission data, the step of frequency correction, and the operation selected from the group consisting of the DC offset operation and the amplitude compensation operation. The modulator is programmed by using a control bus to provide operating parameters to the modulator.
With the foregoing and other objects in view there is also provided, in accordance with the invention, an apparatus for producing spread-coded signals that includes: a spread coding device for coding transmission data using a spread code to obtain spread-coded transmission data, the transmission data being for subsequent transmission; a scrambling device for scrambling the spread-coded transmission data with a scrambling code; and a signal preprocessing device receiving the transmission data from the scrambling device. The signal-preprocessing device includes a device for pulse shaping. The signal-preprocessing device includes a device for frequency correction. The signal-preprocessing device includes a given device for performing an operation selected from the group consisting of a DC offset compensation operation and an amplitude compensation operation. The given device is configured downstream from the device for frequency correction.
In accordance with an added feature of the invention, the device for pulse shaping includes a Root Raised Cosine filter device for pulse shaping.
In accordance with an additional feature of the invention, the Root Raised Cosine filter device has a roll-off factor of approximately 0.22.
In accordance with another feature of the invention, during the pulse shaping process, the Root Raised Cosine filter device uses a specific oversampling factor to oversample the transmission data output from the scrambling device.
In accordance with a further feature of the invention, the device for frequency correction includes a digital frequency correction device. The digital frequency correction device includes microrotation units that are connected in series. The microrotation units use a CORDIC algorithm to correct any frequency offset, and the microrotation units are driven selectively as a function of the frequency offset.
In accordance with a further added feature of the invention, the given device includes a DC offset and amplitude compensation device for performing the operation selected from the group consisting of the DC offset compensation operation and the amplitude compensation operation. The DC offset and amplitude compensation device performs the DC offset compensation operation by adding a specific DC offset compensation value to the transmission data. The DC offset and amplitude compensation device performs the amplitude compensation operation by multiplying the transmission data by a specific amplitude compensation value.
In accordance with a further additional feature of the invention, there is provided, a weighting device multiplying the transmission data by a weighting factor to obtain weighted transmission data. The weighting device is connected upstream from the spread coding device.
In accordance with yet an added feature of the invention, there is provided, an oversampling device configured between the weighting device and the spread coding device. The oversampling device uses a specific sampling factor to oversample the weighted transmission data output from the weighting device.
In accordance with yet an additional feature of the invention, the sampling factor of the oversampling device enables a maximum chip rate to be obtained when the spread coding device subsequently codes the transmission data.
In accordance with yet another feature of the invention, there is provided, a channel adding device configured between the spread coding device and the scrambling device. The transmission data that is being obtained by the weighting device is transmission data from a plurality of physical channels. The weighting device separately weights the transmission data from each one of the physical channels to obtain weighted transmission data for each one of the physical channels. The spread coding device separately codes the weighted transmission data from each one of the physical channels using a channel-specific spread code. After being coded by the spread coding device, the channel adding device combines the transmission data from the physical channels to form an I branch carrying transmission data and a Q branch carrying transmission data. The scrambling device and the signal processing device obtain the transmission data being carried by the I branch separately from the transmission data being carried by the Q branch.
In accordance with yet a further feature of the invention, the apparatus is used in combination with an interface unit and a processor device having a control bus. The interface unit connects the apparatus to the control bus. The processor device uses the interface unit to transmit control data to the apparatus to control operation of the apparatus.
In accordance with an added feature of the invention, the interface unit includes a memory device connected to the apparatus. The memory device is for storing the control data transmitted from the processor device.
In accordance with an additional feature of the invention, the apparatus and the interface unit are formed as hardware, and the processor device is formed as firmware.
In summary, according to the invention, the transmission data is first coded in the normal way using a spread or channelization code and is thus band-spread, and is then coded using a scrambling code. The scrambling process is followed by pulse shaping, frequency correction, and DC offset compensation and/or amplitude compensation that is carried out in the signal path downstream from the frequency correction in order to preprocess the transmission data before it is passed on to a RF interface, from which it is transmitted via a communication channel to a receiver.
RRC filtering with a so-called roll-off factor of approximately 0.22 can be used for pulse shaping, with the band-spread and scrambled transmission data preferably being oversampled using a specific factor, at the same time.
Digital frequency offset correction can be used for frequency correction, using the so-called CORDIC algorithm, while a specific DC offset compensation value is added to the transmission data for DC offset compensation, and a specific amplitude compensation value is multiplied by the transmission data for amplitude compensation.
Before it is coded using the spread code, the transmission data is preferably weighted by being multiplied by a weighting factor, and is then over-sampled. The weighting, oversampling, and spread coding are preferably carried out separately for the transmission data for the various physical channels, in which case, after being coded using the individual spread codes, the transmission data of the physical channels is combined to form an I branch and a Q branch. The transmission data that is carried via the I branch and the Q branch is then scrambled, and is subjected to pulse shaping, frequency correction and DC offset and/or amplitude compensation, which is separately carried out in the signal path downstream from the frequency correction. A factor that corresponds to the highest spread factor for the individual spread codes is preferably used for the oversampling process that is carried out after the weighting process.
A hardware modulator, in particular, is provided for the production and processing, as already described above, of the spread-modulated or spread-coded signals. This modulator accesses various programmable memories or registers in which control data required for operating the modulator is stored. This register is preferably programmed via a control bus by a digital signal processor (DSP) that is in the form of firmware, thus ensuring a high level of flexibility, since further mobile radio standards can also still be included retrospectively just by modifying the software. The programmable registers therefore, allow the functionality of the modulator to be influenced without changing the hardware.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and apparatus for producing spread-coded signals, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.